cp: Minor code cleanup.

• Fix includes in utils.c, cf. style(9).
• Fix type mismatch: readlink(2) returns ssize_t, not int.
• It is not necessary to set errno to 0 as fts_read(3) already does it.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D38369